1. Technical Field
The technical field is methods of planarization. More particularly, the technical field includes planarization methods involving chemical mechanical polishing.
2. Related Art
Planarization is critical in microelectronics processing. Planarization is required in wafer fabrication, shallow trench isolation (STI), dual damascene processes, in the production of flat surfaces for multilevel metal processing, and in many other processes. Chemical mechanical polishing (CMP) is one method of planarization. However, many materials are not suitable for chemical mechanical polishing because they are too soft. CMP processes tend to excessively scratch soft materials, with the polished soft material collecting in the CMP polishing pad and reducing the life of the pad. Soft material also contaminates the CMP pad so that the pad cannot be used to polish wafers having different film materials. Organic spin-on films are one class of materials that generally have good planarization properties, but are regarded as unsuitable for CMP processing.
FIGS. 1A and 1B are process diagrams illustrating a conventional method of planarization. The planarization method can be used to implant a gate using an organic spin-on film. Referring to FIG. 1A, a starting structure includes a patterned wide gate 12 and narrower gates 14 on a substrate 10. An organic film 20 is disposed over the starting structure, and is used to implant the gates 12, 14 independently from the substrate 10, thereby rendering the gates 12, 14 conductive. The film 20 is thin over the gates 12, 14, at regions 22, 26, respectively, and thicker over the substrate 10 between the gates 12, 14, at regions 28. The region 22 is thicker than the region 26.
Referring to FIG. 1B, an etching step is performed to remove the film 20 from over the gates 12, 14, while leaving the remainder of the substrate 10 covered by a resultant film 29. Once the tops of the gates 12, 14 are exposed, the gates 12, 14 can be implanted without implanting the substrate 10, so long as the remaining organic film 29 over the substrate 10 is thick enough to prevent implantation of the substrate 10.
A problem with the above method is that the thickness of the film 20 over the gates 12, 14 is dependent on the length and width of the gates 12, 14 and the surrounding pattern around the gates 12, 14, as shown in FIG. 1A. If a gate''s length and width are too large, a thicker film will be deposited over the larger gate than over a smaller gate. This is illustrated by the thick region 22 over the large gate 12. As shown in FIG. 1B, after etching the film over the small gates 14, some film may remain over the large gate 12. The film over the large gate 12 may prevent some gate implant from entering the large gate 12. Alternatively, if the film over the large gate 12 is completely etched away (not illustrated), the film over the top of the surrounding substrate 10 may be etched too thin to block the gate implant from entering the substrate 10.
The method illustrated in FIGS. 1A and 1B illustrate problems arising from nonuniform thicknesses of spin-on materials over gates that are to be implanted. However, similar difficulties arise in other situations where structures with topography are covered with spin-on materials and the structures are selectively exposed and subjected to further processing.